Kerslake et al. (1998) collected five-year-old heather plants from a Scottish moor and grew them in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm for 20 months, with and without soil nitrogen fertilization. At two different times during the study, larvae of Operophtera brumata, a voracious winter moth whose outbreaks have caused extensive damage to heather moorland in recent years, were allowed to feed upon current-year shoots for up to one month. The results obtained from this experiment revealed that the survivorship of larvae placed on CO2-enriched foliage was not significantly different from that of larvae placed on foliage produced in ambient air, regardless of nitrogen treatment. In addition, feeding upon CO2-enriched foliage did not affect larval growth rate, development or final pupal weight. Consequently, Kerslake et al. concluded that their study "provides no evidence that increasing atmospheric CO2 concentrations will affect the potential for outbreak of Operophtera brumata on this host."

Hattenschwiler and Schafellner (1999) grew seven-year-old spruce trees at atmospheric CO2 concentrations of 280, 420 and 560 ppm in various nitrogen deposition treatments for three years, after which they performed needle quality assessments and allowed nun moth (Lymantria monacha) larvae to feed upon current-year needles for twelve days. This moth is an especially voracious defoliator that resides in most parts of Europe and East Asia between 40 and 60° N latitude, and it is commonly regarded as the "coniferous counterpart" of its close relative the gypsy moth, which feeds primarily upon deciduous trees.

The two scientists determined from their observations that elevated CO2 significantly enhanced needle starch, tannin and phenolic concentrations, while significantly decreasing needle water and nitrogen contents. Thus, atmospheric CO2 enrichment reduced overall needle quality from the perspective of this foliage-consuming moth, as nitrogen content is the primary factor associated with leaf quality. Increasing nitrogen deposition, on the other hand, tended to enhance needle quality, for it lowered starch, tannin and phenolic concentrations while boosting needle nitrogen content. Nevertheless, the positive influence of nitrogen deposition on needle quality was not large enough to completely offset the quality reduction caused by elevated CO2.

In light of these observations, it was no surprise that larvae placed on CO2-enriched foliage consumed less needle biomass than larvae placed on low-CO2-grown foliage, regardless of nitrogen treatment, and that the larvae feeding on CO2-enriched foliage exhibited reduced relative growth rates and attained an average biomass that was only two-thirds of that attained by larvae consuming foliage produced at 280 ppm CO2. As a result, Hattenschwiler and Schafellner concluded that "altered needle quality in response to elevated CO2 will impair the growth and development of Lymantria monacha larvae," which should lead to reductions in the degree of spruce tree destruction caused by this voracious defoliator.

Stiling et al. (2002) studied the effects of an approximate doubling of the air's CO2 concentration on a number of characteristics of several insect herbivores feeding on plants native to a scrub-oak forest ecosystem at the Kennedy Space Center, Florida, USA, in eight ambient and eight CO2-enriched open-top chambers. In describing their findings, they say that the "relative levels of damage by the two most common herbivore guilds, leaf-mining moths and leaf-chewers (primarily larval lepidopterans and grasshoppers), were significantly lower in elevated CO2 than in ambient CO2," and that "the response to elevated CO2 was the same across all plant species." Also, they report that "more host-plant induced mortality was found for all miners on all plants in elevated CO2 than in ambient CO2." These effects were so powerful, in fact, that in addition to the relative densities of insect herbivores being reduced in the CO2-enriched chambers, and "even though there were more leaves of most plant species in the elevated CO2 chambers," the total densities of leaf miners in the high-CO2 chambers were also lower for all plant species. Hence, it would appear that in a higher CO2 world of the future, earth's natural ecosystems may well be able to better withstand the onslaughts of various insect pests, including moths, that have plagued them in years and ages past. An interesting implication of this finding, as Stiling et al. note, is that "reductions in herbivore loads in elevated CO2 could boost plant growth beyond what might be expected based on pure plant responses to elevated CO2 [our italics]," which is a truly exciting observation.

In a follow-up study to that of Stilling et al., which was conducted at the same facilities, Rossi et al. (2004), focused on the abundance of a guild of lepidopteran leafminers that attack the leaves of myrtle oak, as well as various leaf chewers that also like to munch on this species. Specifically, they periodically examined 100 marked leaves in each of the sixteen open-top chambers for a total of nine months, after which, in their words, "differences in mean percent of leaves with leafminers and chewed leaves on trees from ambient and elevated chambers were assessed using paired t-tests." This protocol revealed, in their words, that "both the abundance of the guild of leafmining lepidopterans and damage caused by leaf chewing insects attacking myrtle oak were depressed in elevated CO2." Leafminer abundance was 44% lower (P = 0.096) in the CO2-enriched chambers compared to the ambient-air chambers, while the abundance of leaves suffering chewing damage was 37% lower (P = 0.072) in the CO2-enriched air.

Working with red maple saplings, Williams et al. (2003) bagged first instar gypsy moth larvae on branches of trees that were entering their fourth year of growth within open-top chambers maintained at four sets of CO2/temperature conditions: (1) ambient temperature, ambient CO2, (2) ambient temperature, elevated CO2 (ambient + 300 ppm), (3) elevated temperature (ambient + 3.5°C), ambient CO2, and (4) elevated temperature, elevated CO2. For these conditions they measured several parameters that were required to test their hypothesis that a CO2-enriched atmosphere would lead to reductions in foliar nitrogen concentrations and increases in defensive phenolics that would in turn lead to increases in insect mortality. The results they obtained indicated, in their words, "that larvae feeding on CO2-enriched foliage ate a comparably poorer food source than those feeding on ambient CO2-grown plants, irrespective of temperature." Nevertheless, they determined that "CO2-induced reductions in foliage quality were unrelated to insect mortality, development rate and pupal weight." As a result, they were forced to conclude that "phytochemical changes resulted in no negative effects on gypsy moth performance," but neither did they help them.

Noting that increases in the atmosphere's CO2 concentration typically lead to greater decreases in the concentrations of nitrogen in the foliage of C3 as opposed to C4 grasses, Barbehenn et al. (2004) say "it has been predicted that insect herbivores will increase their feeding damage on C3 plants to a greater extent than on C4 plants (Lincoln et al., 1984, 1986; Lambers, 1993). To test this hypothesis, they thus grew Lolium multiflorum (Italian ryegrass, a common C3 pasture grass) and Bouteloua curtipendula (sideoats gramma, a native C4 rangeland grass) in chambers maintained at either the ambient atmospheric CO2 concentration of 370 ppm or the doubled CO2 concentration of 740 ppm for two months, after which newly-molted sixth-instar larvae of Pseudaletia unipuncta (a grass-specialist noctuid) and Spodoptera frugiperda (a generalist noctuid) were allowed to feed upon the two grasses.

As expected, Barbehenn et al. found that foliage protein concentration decreased by 20% in the C3 grass, but by only 1% in the C4 grass, when they were grown in CO2-enriched air; and they say that "to the extent that protein is the most limiting of the macronutrients examined, these changes represent a decline in the nutritional quality of the C3 grass." However, and "contrary to our expectations," in the words of Barbehenn et al., "neither caterpillar species significantly increased its consumption rate to compensate for the lower concentration of protein in [the] C3 grass," and they note that "this result does not support the hypothesis that C3 plants will be subject to greater rates of herbivory relative to C4 plants in future [high-CO2] atmospheric conditions (Lincoln et al., 1984)." In addition, and "despite significant changes in the nutritional quality of L. multiflorum under elevated CO2," they note that "no effect on the relative growth rate of either caterpillar species on either grass species resulted," and that there were "no significant differences in insect performance between CO2 levels." By way of explanation of these results, they suggest that "post-ingestive mechanisms could provide a sufficient means of compensation for the lower nutritional quality of C3 plants grown under elevated CO2."

In light of these observations, and contrary to early simplistic thought on the matter, Barbehenn et al. suggest "there will not be a single pattern that characterizes all grass feeders" with respect to their feeding preferences and developmental responses in a world where certain C3 plants may experience foliar protein concentrations that are lower than those they exhibit today, nor will the various changes that may occur necessarily be detrimental to herbivore development or to the health and vigor of their host plants. Nevertheless, subsequent studies continue to suggest that various moth species will likely be negatively impacted by the ongoing rise in the air's CO2 content.

A case in point is the study of Chen et al. (2005), who grew well watered and fertilized cotton plants of two varieties (one expressing Bacillus thurigiensis toxin genes and one a non-transgenic cultivar from the same recurrent parent) in pots placed within open-top chambers maintained at either 376 or 754 ppm CO2 in Sanhe County, Hebei Province, China, from planting in mid-May to harvest in October, while immature bolls were periodically collected and analyzed for various chemical characteristics and others were stored under refrigerated conditions for later feeding to larvae of the cotton bollworm. By these means they found that the elevated CO2 treatment increased immature boll concentrations of condensed tannins by approximately 22% and 26% in transgenic and non-transgenetic cotton, respectively, and that it slightly decreased the body biomass of the cotton bollworm and reduced moth fecundity. The Bt treatment was even more effective in this regard; and in the combined Bt-high-CO2 treatment, the negative cotton bollworm responses were expressed most strongly of all.

Bidart-Bouzat et al. (2005) grew three genotypes of mouse-ear cress (Arabidopsis thaliana) from seed in pots within controlled-environment chambers maintained at either ambient CO2 (360 ppm) or elevated CO2 (720 ppm). On each of half of the plants (the herbivory treatment) in each of these CO2 treatments, they placed two second-instar larvae of the diamondback moth (Plutella xylostella) at bolting time and removed them at pupation, which resulted in an average of 20% of each plant's total leaf area in the herbivory treatment being removed. Then, each pupa was placed in a gelatin capsule until adult emergence and ultimate death, after which insect gender was determined and the pupa's weight recorded. At the end of this herbivory trial, the leaves of the control and larvae-infested plants were analyzed for concentrations of individual glucosinolates -- a group of plant-derived chemicals that can act as herbivore deterrents (Maruicio and Rausher, 1997) -- while total glucosinolate production was determined by summation of the individual glucosinolate assays. Last of all, influences of elevated CO2 on moth performance and its association with plant defense-related traits were evaluated.

Overall, it was determined by these means that herbivory by larvae of the diamondback moth did not induce any increase in the production of glucosinolates in the mouse-ear cress in the ambient CO2 treatment. However, the three scientists report that "herbivory-induced increases in glucosinolate contents, ranging from 28% to 62% above basal levels, were found under elevated CO2 in two out of the three genotypes studied." In addition, they determined that "elevated CO2 decreased the overall performance of diamondback moths." And because "induced defenses can increase plant fitness by reducing subsequent herbivore attacks (Agrawal, 1999; Kessler and Baldwin, 2004)," according to Bidart-Bouzat et al., they suggest that "the pronounced increase in glucosinolate levels under CO2 enrichment may pose a threat not only for insect generalists that are likely to be more influenced by rapid changes in the concentration of these chemicals, but also for other insect specialists more susceptible than diamondback moths to high glucosinolate levels (Stowe, 1998; Kliebenstein et al., 2002)." Hence, it is tempting to speculate that the ongoing rise in the air's CO2 content will enable earth's vegetation to better withstand the ravages of marauding herbivores in the years and decades ahead.

In a study of a major crop species, Wu et al. (2006) grew spring wheat (Triticum aestivum L.) from seed to maturity in pots placed within open-top chambers maintained at either 370 or 750 ppm CO2 in Sanhe County, Hebei Province, China, after which they reared three generations of cotton bollworms (Helicoverpa armigera Hubner) on the milky grains of the wheat, while monitoring a number of different bollworm developmental characteristics. In doing so, as they describe it, "significantly lower pupal weights were observed in the first, second and third generations," and "the fecundity of H. armigera decreased by 10% in the first generation, 13% in the second generation and 21% in the third generation," resulting in a "potential population decrease in cotton bollworm by 9% in the second generation and 24% in the third generation." In addition, they say that "population consumption was significantly reduced by 14% in the second generation and 24% in the third generation," and that the efficiency of conversion of ingested food was reduced "by 18% in the first generation, 23% in the second generation and 30% in the third generation." As a result, they concluded that the "net damage of cotton bollworm on wheat will be less under elevated atmospheric CO2," while noting that "at the same time, gross wheat production is expected to increase by 63% under elevated CO2."

In another report of their work, Wu et al. (2007) write that "significant decreases in the protein, total amino acid, water and nitrogen content by 15.8%, 17.7%, 9.1% and 20.6% and increases in free fatty acid by 16.1% were observed in cotton bolls grown under elevated CO2." And when fed with these cotton bolls, they say that the larval survival rate of H. armigera "decreased by 7.35% in the first generation, 9.52% in the second generation and 11.48% in the third generation under elevated CO2 compared with ambient CO2." In addition, they observed that "the fecundity of H. armigera decreased by 7.74% in the first generation, 14.23% in the second generation and 16.85% in the third generation," while noting that "fecundity capacity is likely to be reduced even further in the next generation."

The synergistic effects of these several phenomena, in the words of Wu et al., "resulted in a potential population decrease in cotton bollworm by 18.1% in the second generation and 52.2% in the third generation under elevated CO2," with the result that "the potential population consumption of cotton bollworm decreased by 18.0% in the second generation and 55.6% in the third generation ... under elevated CO2 compared with ambient CO2." And in light of these several findings, they concluded that "the potential population dynamics and potential population consumption of cotton bollworm will alleviate the harm to [cotton] plants in the future rising-CO2 atmosphere."

In a different type of study, Esper et al. (2007) reconstructed an annually-resolved history of population cycles of a foliage-feeding Lepidopteran commonly known as the larch budmoth (Zeiraphera diniana Gn.) -- or LBM for short -- within the European Alps in the southern part of Switzerland. As is typical of many such insect pests, they note that "during peak activity, populations may reach very high densities over large areas," resulting in "episodes of massive defoliation and/or tree mortality" that could be of great ecological and economic significance.

The first thing the team of Swiss and US researchers thus did in this regard was develop a history of LBM outbreaks over the 1173-year period AD 832-2004, which they describe as "the longest continuous time period over which any population cycle has ever been documented." They accomplished this feat using radiodensitometric techniques to characterize the tree-ring density profiles of 180 larch (Larix deciduas Mill.) samples, where "LBM outbreaks were identified based upon characteristic maximum latewood density (MXD) patterns in wood samples, and verified using more traditional techniques of comparison with tree-ring chronologies from non-host species," i.e., fir and spruce. Then, they developed a matching temperature history for the same area, which was accomplished by combining "a tree-ring width-based reconstruction from AD 951 to 2002 integrating 1527 pine and larch samples (Buntgen et al., 2005) and a MXD-based reconstruction from AD 755 to 2004 based upon the same 180 larch samples used in the current study for LBM signal detection (Buntgen et al., 2006)."

Over almost the entire period studied, i.e., from its start in AD 832 to 1981, there were a total of 123 LBM outbreaks with a mean reoccurrence time of 9.3 years. In addition, the researchers say "there was never a gap that lasted longer than two decades." From 1981 to the end of their study in 2004, however, there were no LBM outbreaks; and since there had never before (within their record) been such a long outbreak hiatus, they concluded that "the absence of mass outbreaks since the 1980s is truly exceptional."

To what do Esper et al. attribute this unprecedented recent development? Noting that "conditions during the late twentieth century represent the warmest period of the past millennium" -- as per their temperature reconstruction for the region of the Swiss Alps within which they worked -- they point to "the role of extraordinary climatic conditions as the cause of outbreak failure," and they discuss what they refer to as the "probable hypothesis" of Baltensweiler (1993), who described a scenario by which local warmth may lead to reduced LBM populations.

Such may well be the case; but we hasten to add that atmospheric CO2 concentrations since 1980 have also been unprecedented over the 1173-year period of Esper et al.'s study. In fact, they have been even more unprecedented than have air temperatures. Hence, the suppression of LBM outbreaks over the past quarter-century may possibly have been the result of some synergistic consequence of the two factors (temperature and CO2) acting in unison, while a third possibility may involve only the increase in the air's CO2 content.

Whatever the case may be, Esper et al. say their findings highlight the "vulnerability of an otherwise stable ecological system in a warming environment," in what would appear to be an attempt to attach an undesirable connotation to the observed outcome. This wording seems strange indeed, for it is clear that the "recent disruption of a major disturbance regime," as Esper et al. refer to the suppression of LBM outbreaks elsewhere in their paper, would be considered by most people to be a positive outcome, and something to actually be welcomed.

Working with Antheraea polyphemus -- a leaf-chewing generalist lepidopteran herbivore that represents the most abundant feeding guild in the hardwood trees that grow beneath the canopy of the unmanaged loblolly pine plantation that hosts the Forest Atmosphere Carbon Transfer and Storage (FACTS-1) research site in the Piedmont region of North Carolina, USA, where the leaf-chewer can consume 2-15% of the forest's net primary production in any given year -- Knepp et al. (2007) focused their attention on two species of oak tree -- Quercus alba L. (white oak) and Quercus velutina Lam. (black oak) -- examining host plant preference and larval performance of A. polyphemus when fed foliage of the two tree species that had been grown in either ambient or CO2-enriched air (to 200 ppm above ambient) in this long-running FACE experiment. In doing so, they determined that "growth under elevated CO2 reduced the food quality of oak leaves for caterpillars," while "consuming leaves of either oak species grown under elevated CO2 slowed the rate of development of A. polyphemus larvae." In addition, they found that feeding on foliage of Q. velutina that had been grown under elevated CO2 led to reduced consumption by the larvae and greater mortality. As a result, they concluded that "reduced consumption, slower growth rates, and increased mortality of insect larvae may explain [the] lower total leaf damage observed previously in plots of this forest exposed to elevated CO2," as documented by Hamilton et al. (2004) and Knepp et al. (2005), which finding bodes well indeed for the growth and vitality of such forests in the years and decades ahead, as the air's CO2 content continues to rise.

Kampichler et al. (2008) also worked with oak trees. Noting, however, that "systems studied so far have not included mature trees," they attempted to remedy this situation by determining "the abundance of dominant leaf-galls (spangle-galls induced by the cynipid wasps Neuroterus quercusbaccarum and N. numismalis) and leaf-mines (caused by the larvae of the moth Tischeria ekebladella) on freely colonized large oaks in a mixed forest in Switzerland, which received CO2 enrichment [540 ppm vs. 375 ppm during daylight hours] from 2001 to 2004" via "the Swiss Canopy Crane (SCC) and a new CO2 enrichment technique (web-FACE)" in a forest that they say "is 80-120 years old with a canopy height of 32-38 m, consisting of seven deciduous and four coniferous species." This work allowed the German, Mexican and Swiss researchers to discover that although elevated CO2 reduced various leaf parameters (water content, proteins, non-structural carbohydrates, tannins, etc.) at the SCC site, "on the long term, their load with cynipid spangle-galls and leaf-mines of T. ekebladella was not distinguishable from that in oaks exposed to ambient CO2 after 4 years of treatment." Consequently, although speculation has run rampant over the years about the long-term effects of atmospheric CO2 enrichment on plant foliage and its subsequent effects on animals of various trophic levels, Kampichler et al. concluded that in the situation they investigated, "CO2 enrichment had no lasting effect in all three [animal] taxa, despite the substantial and consistent change in leaf chemistry of oak due to growth in elevated CO2."

In conclusion, therefore, and considering the results of all of the studies reviewed in this summary, it would appear that the ongoing rise in the air's CO2 content will not result in greater damage to earth's vegetation by the larvae of the many moths that inhabit the planet. If anything, it could well reduce the damage they cause.

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